Magnetic recording medium

ABSTRACT

A magnetic recording medium including a non-magnetic base and a magnetic layer thereon which contains and/or is coated with an organopolysiloxane having an average unit with the formula: exhibits 
     
         (CH.sub.3)(RO).sub.n (R&#39;COO).sub.m SiO.sub.(3-n-m)/2 
    
     wherein R is a monovalent hydrocarbon group having from 1 to 5 carbon atoms, R&#39; is an aliphatic monovalent hydrocarbon group having from 7 to 17 carbon atoms; n is zero or a positive number, m is a positive number with the proviso that n + m is less than 3 and the number of Si atoms in a molecule of such organopolysiloxane ranges from 2 to 8. Magnetic recording medium containing such organopolysiloxane material exhibit a substantially reduced dynamic friction coefficient and a substantially reduced tendency of the magnetic layer to lose magnetic particles, along with an improved splicing-ability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic recording medium having anon-magnetic base and a magnetic layer thereon containing magnetizableparticles dispersed in a resinous binder in combination with alubricant.

2. Prior Art

A magnetic recording medium, whether used for audio recording, videorecording or other magnetic recording purposes comes in contact withguide members, magnetic heads and the like during use. For example, in acase of a video tape recorder, where high tape velocities areencountered, the tape must have sufficient wear resistance and arelatively small friction coefficient if it is to run smoothly andsteadily for a long time. Moreover, the magnetic powder on a recordingmedium must be sufficiently bound thereto to resist shedding orloosening of such powder during use and in instances where the recordingmedium is tape, it must exhibit good splicing-ability.

Magnetic recording media which have an increased friction coefficientvibrate at the guide members and at the magnetic heads during therecording operation and/or during the reproducing operation so that therecorded signals or the reproduced signals are distorted from theoriginal signals. In some cases, a so-called "Q" sound, due to thevibration of the magnetic recording media, is encountered.

Efforts have been made to overcome the above-described defects and toimpart lubricity or smoothness to the magnetic recording media, but nocompletely satisfactory lubricant for magnetic recording media, has yetbeen developed. For example, it has been suggested to use lubricants,such as silicone fluid, castor oil, molybdenum disulfide, graphite,higher fatty acids and the like whereby the lubricant is mixed into amagnetic layer containing a magnetic powder, such as gamma ferric oxide,α -Fe₂ O₃, and a binder, such as polyvinyl chloride. Magnetic recordingmedia containing such lubricants exhibit some wear resistance, but notto a sufficient degree. When a large quantity of one of the aboveenumerated lubricants is mixed into the magnetic layer in order tofurther increase the wear resistance, so-called "bleeding" or "blooming"phenomena occur on the magnetic layer. The bleeding or bloomingphenomena result from the lubricating agent oozing or diffusing onto thesurface of the magnetic layer and becoming separated therefrom. As aresult, the surface of the magnetic recording media gets rough and moremagnetic powder separates or is rubbed off from the magnetic recordinglayer during use. When the friction coefficient is too large, the "Q"sound cannot be avoided.

Recently, S. Higuchi et al disclosed and claimed in U.S. Pat. No.3,993,846 a polyoxyalkylene substituted organosilicon compound havingthe formula:

    RO(CHR"CH.sub.2 O).sub.n.sbsb.1 (SiCH.sub.3 CH.sub.3 O).sub.m (CH.sub.2 CHR"O).sub.n.sbsb.2 R'

wherein R and R' comprise an aliphatic hydrocarbon group having from 8to 18 carbon atoms, R" is a hydrogen atom or a methyl group, m is aninteger ranging from 1 to 15 and n₁ and n₂ are integers whose sums rangefrom 2 to 16 as a lubricant for a magnetic recording medium. Thislubricant exhibits considerable promise in providing an improvedmagnetic recording medium wherein at least some, if not all, of theaforesaid prior art problems are alleviated. S. Higuchi et al alsodisclosed and claimed in U.S. Pat. No. 4,007,313 a fluoro-organosiliconcompound having the formula: ##STR1## wherein R is an aliphatichydrocarbon group having from 7 to 17 carbon atoms and n is an integerranging from 1 to 3 as a lubricant for use with the magnetic recordingmedium. This lubricant also exhibits considerable promise in providingan improved magnetic recording medium wherein at least some, if not all,of the aforesaid prior art problems are alleviated. S. Higuchi et alalso disclosed and claimed in U.S. Pat. No 4,007,314 an organosiliconcompound having the formula:

    (RCOO).sub.n Si(CH.sub.3).sub.4-n

wherein R is an aliphatic hydrocarbon group containing from 7 to 17carbon atoms and n is an integer ranging from 1 to 3 as a lubricant foruse with a magnetic recording medium. This lubricant also exhibitsconsiderable promise in providing an improved magnetic recording mediumwherein at least some, if not all, of the aforesaid prior art problemsare alleviated. While the aforesaid Higuchi et al lubricants arepreferred over prior art lubricants, it is nevertheless desirable todevelope yet further improved magnetic recording lubricants.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, a magneticrecording medium is provided with a non-magnetic base and a magneticlayer formed thereon which includes magnetizable particles dispersed ina resinous binder and a sufficient amount of lubricant to providelubricating properties to such magnetic layer.

The lubricant used in the practice of the invention comprises anorganopolysiloxane material expressed by the following average unitformula:

    (CH.sub.3)(RO).sub.n (R'COO).sub.m SiO.sub.(3-n-m)/2

wherein R is a monovalent hydrocarbon group having from 1 to 5 carbonatoms, R' is an aliphatic monovalent hydrocarbon group having from 7 to17 carbon atoms, n is zero or a positive number, m is a positive numberwith the proviso that the sum of n and m is less than 3 and the numberof Si atoms within a molecule of such organopolysiloxane ranges from 2to 8.

The lubricant of the invention may be intermixed into a magnetic layerand/or the lubricant may be coated onto a magnetic layer. In preferredembodiments, the amount of lubricant intermixed into a magnetic layerranges from about 0.5 to 6 parts by weight per 100 parts by weight ofmagnetizable particles within the magnetic layer and the amount oflubricant coated onto a magnetic layer ranges from about 30 to 1500milligrams per square meter of magnetic layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic view illustrating an arrangement formeasuring the dynamic friction coefficient, μ d, of a magnetic recordingtape produced in accordance with the principles of the invention;

FIG. 2 is a somewhat schematic view illustrating an arrangement formeasuring the splicing-ability of a magnetic recording tape produced inaccordance with the principles of the invention; and

FIG. 3 is a somewhat schematic elevational view illustrating anembodiment of a magnetic recording medium produced in accordance withthe principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides an improved magnetic recording medium comprisedof a non-magnetic base having a magnetic layer thereon comprised ofmagnetic or magnetizable particles dispersed in a resinous binder incombination with a sufficient amount of an organopolysiloxane lubricantto provide lubricating properties to such magnetic layer.

Magnetic recording media, such as flexible magnetic recording tapes orrelatively rigid magnetic recording discs, produced in accordance withthe principles of the invention, exhibit superior wear-resistancecharacteristics relative to prior art magnetic recording media. Thefriction coefficient of magnetic recording media produced in accordancewith the principles of the invention is considerably reduced and thelubricity or smoothness thereof is considerably improved.

Accordingly, magnetic recording media produced in accordance with theprinciples of the invention run smoothly and steadily over guidemembers, magnetic heads and the like for a relatively long time. Withmagnetic recording medium produced in accordance with the principles ofthe invention, little, if any, "Q" sound is generated. Further, theamount of magnetic powder rubbed off during use of a magnetic recordingmedium produced in accordance with the principles of the invention isremarkably reduced. In addition, the splicing-ability of magneticrecording tapes produced in accordance with the principles of theinvention is remarkably improved.

The organopolysiloxanes used in the practice of the invention arecomprised of molecules expressed by the following average unit formula:

    (CH.sub.3)(RO).sub.n (R'COO).sub.m SiO.sub.(3-n-m)/2       (I)

wherein R is a monovalent hydrocarbon group having from 1 to 5 carbonatoms; R' is an aliphatic monovalent hydrocarbon group having from 7 to17 carbon atoms; n is zero or a positive number; m is a positive numberwith the proviso that the sum of n and m is less than 3; and the numberof Si atoms in a molecule of such organopolysiloxane material rangesfrom 2 to 8. As will be appreciated, in silicone chemistry, it isconventional to represent structural formulas of silicon compounds basedon a single silicon atom so that the organopolysiloxane materials usedin the present invention may be defined by the average unit formula (I)above.

The organopolysiloxanes used in the practice of the invention may belinear, branched or cyclic in molecular configuration. Suchorganopolysiloxanes have at least one acyloxy group, R'COO--, within amolecule which, optionally, is in combination with one or more alkoxygroups, RO--. The R' within an acyloxy group is a long-chainedmonovalent saturated or unsaturated hydrocarbon group and in instanceswhere two or more acyloxy groups are present in an organopolysiloxanemolecule, the R' portion thereof may be the same or different. The Rwithin an alkoxy group is a monovalent saturated or unsaturatedhydrocarbon group and in instances where two or more alkoxy groups arepresent within a molecule, the R portion thereof may be the same ordifferent.

The number of carbon atoms in an R of an alkoxy group preferably rangesfrom 1 to 5, because an organopolysiloxane having an alkoxy grouptherein with more than 5 carbon atoms does not yield practicalimprovements in splicing-ability and tends to cause an unsatisfactorywear resistance of a magnetic recording medium provided with such acompound.

The number of carbon atoms in an R' of an acyloxy group preferablyranges from 7 to 17, because an organopolysiloxane having an acyloxygroup with less than 7 carbon atoms tends to cause an increase in thefriction coefficient of magnetic recording medium provided with suchcompound. On the other hand, an organopolysiloxane having an acyloxygroup with more than 17 carbon atoms tends to cause excessive bleedingor blooming and tends to increase the amount of magnetic particlesseparating or rubbing-off during use of a magnetic recording mediumprovided with such compound.

Each molecule of the organopolysiloxanes used in the practice of theinvention contains from 2 to 8 silicon atoms. In instances where thenumber of Si atoms within an organopolysiloxane is greater than 8, thefriction coefficient of a magnetic recording medium provided with suchorganopolysiloxane molecule is too large and the lubricity of suchrecording medium is not improved. On the other hand, when the number ofSi atoms within a molecule is 1, the compound no longer comprises anorganopolysiloxane but instead comprises an organosilane and thefriction coefficient of a magnetic recording medium provided with suchorganosilane is too large so that satisfactory wear-resistance andlubricity cannot be attained therewith.

In accordance with the principles of the invention, theorganopolysiloxane lubricants may be added or intermixed with themagnetic particles and binders used in forming a magnetic layer or theorganopolysiloxane lubircant may be coated onto a magnetic layer as atop coat or the magnetic layer may contain the organopolysiloxanestherein and also be coated with a layer of such organopolysiloxane. Inembodiments where the lubricant is added within a magnetic layer, theamount of organopolysiloxane ranges from about 0.5 to about 6 parts byweight, per 100 parts by weight of a magnetic powder, such as gammaferric oxide. In instances where more than 6 parts by weight (based on100 parts by weight of magnetic particles) of an organopolysiloxane areadded to a magnetic powder, the adhesive strength of the resultingmagnetic layer is reduced so that the magnetic powder tends to rub offmore readily during use, although the friction coefficient of suchmagnetic layer is reduced. In instances where less than 0.5 parts byweight of an organopolysiloxane are used, insufficient lubricity isprovided to the magnetic recording medium and the friction coefficientis insufficiently reduced. In embodiments where the lubricant istop-coated onto a magnetic layer, the lubricant layer is applied so thatabout 30 to 1500 milligrams of organopolysiloxane is present per squaremeter of magnetic layer and preferably the amount of organopolysiloxanein a top coat on a magnetic layer ranges from about 60 to 700 milligramsper square meter.

The magnetic powder or magnetizable particles used in magnetic recordingmedia produced in accordance with the invention may be composed of anyavailable magnetic or magnetizable material, such as gamma hematite (α-Fe₂ O₃); magnetite (Fe₃ O₄); gamma hematite or magnetite doped withnon-ferrous atoms, such as cobalt; iron oxides of non-stoichiometricoxidation compounds between gamma hematite and magnetite; chromiumdioxide (CrO₂); magnetic or magnetizable alloys, such as an iron-cobaltalloy (Fe-Co); and iron-cobalt-nickel alloy (Fe-Co-Ni); mixtures of theabove or other magnetic or magnetizable materials.

The resinous binder used in magnetic recording media produced inaccordance with the principles of the invention may be anyconventionally useful resinous binder, such as vinyl chloride-vinylacetate copolymer resins; vinyl polyurethane resins; epoxy resins;phenoxy resins; polyester resins; mixtures thereof or other likeresinous binders.

Magnetic recording media produced in accordance with the principles ofthe invention may also include antistatic agents of the typeconventionally used with magnetic recording media. An example of asuitable antistatic agent is carbon black but other suitable antistaticagents may also be used. Further, dispersing agents, such as lecithinand the like, may be added to a magnetic layer of a magnetic recordingmedium in accordance with conventional practices.

The magnetic recording medium of the invention may comprise any of theknown forms, such as magnetic recording tapes having a flexiblenon-magnetic film base, discs having a relatively rigid non-magneticbase such as composed of a ceramic or a metal, for example, aluminum.

The organopolysiloxane compounds used in the practice of the inventionmay be synthesized in relatively high yields via various routes. Forexample, an ester interchange reaction may be effected whereinmethylalkoxy polysiloxane having an average unit formula:

    (CH.sub.3)(RO).sub.n' SiO.sub.(3-n')/2                     (II)

wherein R is a monovalent hydrocarbon group having from 1 to 5 carbonatoms, n' is a positive number less than 3, and the number of Si atomsin a molecule of the methylalkoxy polysiloxane ranges from 2 to 8 isreached with a fatty acid having the formula R'COOH, wherein R' is analiphatic monovalent hydrocarbon group having from 7 to 17 carbon atoms.The methylalkoxy polysiloxane (II) described above, may be obtained by acondensation reaction of the partial hydrolysis product ofmethyltrialkoxysilane, which may be represented by the formula CH₃Si(OR)₃ wherein R is identical with R in formula (II) above. The aboveester interchange reaction may be expressed by the following equation:

    (CH.sub.3)(RO).sub.n' SiO.sub.(3-n'/2 +mR' COOH→(CH.sub.3)(RO).sub.n (R'COO).sub.m SiO.sub.(3-n-m)/2 +mROH                     (III)

wherein R and R' are the same as above and n' is equal to the sum of mand n.

Another exemplary synthesis route for attaining the organopolysiloxanecompounds of the invention is a dehydrochlorination reaction whereinmethylchloropolysiloxane expressed by the following average unitformula:

    (CH.sub.3)(Cl).sub.n' SiO.sub.(3-n')/ 2                    (IV)

wherein n' is a positive number less than 3 and the number of Si atomsin a molecule of the methylchloropolysiloxane ranges from 2 to 8 isreacted with an alcohol having the formula ROH wherein R is a monovalenthydrocarbon group having from 1 to 5 carbon atoms and with a fatty acidhaving the formula R'COOH wherein R' is an aliphatic monovalenthydrocarbon group having from 7 to 17 carbon atoms. Themethylchloropolysiloxane (IV) described above may be obtained by acondensation reaction of the partial hydrolysis product ofmethyltrichlorosilane, which has the formula CH₃ SiCl₃. The abovedehydrochlorination reaction may be expressed by the following equation:##STR2## wherein R and R' are the same as above and n' is equal to thesum of n and m (* preferably pyridine or another suitabledehydrochlorination agent).

With the foregoing general discussion in mind, the following specificexamples are presented to further illustrate to those skilled in the artthe manner in which the invention is carried out. However, the examplesare not to be construed as limiting the scope of the invention in anyway.

EXAMPLE I

In this exemplary embodiment of the invention, six samples of the abovedescribed organopolysiloxanes were prepared in an essentially identicalmanner so that the synthesis route for the organopolysiloxane ininventive sample No. 1 will only be described in detail.

316 Grams (one mole) of a methylmethoxypolysiloxane, obtained by thepartial hydrolysis-condensation of CH₃ Si(OCH₃)₃ and which has thefollowing formula: ##STR3## and 316 grams (2 moles) of pelargonic acid,C₈ H₁₇ COOH, were added into 500 grams of toluene. 2.0 Grams oftrifluoroacetic acid were added as a catalyst into the above mixture andthe mixture was then heated, with stirring. The methanol, which formedduring the substitution reaction, was distilled azeotropically from themixture, along with toluene. When about a theoretical amount of methanolwas removed, the mixture was cooled and the catalyst was neutralizedwith sodium carbonate, Na₂ CO₃. The mixture was then filtered and theremaining solvent was removed. In this manner, 528 grams of anorganopolysiloxane expressed by the following average unit formula:(CH₃)(CH₃ O)(C₈ H₁₇ COO)_(2/3) SiO_(2/3) (i.e., a molecule of thiscompound had 3 silicon atoms therein), was obtained. Thisorganopolysiloxane was a light-yellow liquid having a viscosity of 25 cSat 25° C. and a yield of about 93% theoretical was obtained.

The organopolysiloxanes specified in inventive samples 2, 3, 4, 5 and 6as shown in Table I below were obtained in a similar manner as thatdescribed above.

Two parts by weight (based on 100 parts by weight of magnetic powder) ofeach of the organopolysiloxanes shown in Table I were added into amagnetic composition having the following ingredients (wherein allamounts shown are by weight):

    ______________________________________                                        Ferromagnetic chromium dioxide powder                                                                  100    parts                                         Saturated Polyester resin.sup.1                                                                        10.0   parts                                         Phenoxy resin.sup.2      10.0   parts                                         Polyurethane resin.sup.3 10.0   parts                                         Dispersing agent.sup.4   1.0    part                                          Methylethylketone        150    parts                                         Methylisobutylketone     150    parts                                         ______________________________________                                         .sup.1 a commercially available saturated polyester resin available under     the trade name "Vitel PE-200" from Goodyear Tire & Rubber Corp.               .sup.2 a commercially available phenoxy resin available under the trade       name "Bakelite Phenoxy Resin PKHH" from Union Carbide Corp.                   .sup.3 a commercially available polyurethane resin available under the        trade name "Estane 5702" from B. F. Goodrich                                  .sup.4 a commercially available dispersing agent comprised of                 N-tallowoxy-trimethylenediamine available under the trade name "Duomeen T     from Armour & Chemical Corp.                                             

The above magnetic composition containing an organopolysiloxane thereinwas then mixed with a polyisocyanate compound (for example, acommercially available polyisocyanate, such as comprised of the reactionproduct of 1 mole of 1,1,1 -trimethylolpropane and 3 moles of2,4-toluenediisocyanate, available under the trade name "Desmodur L-75"which comprises a mixture containing about 75% of the above reactionproduct and about 25% of a solvent, such as ethyl acetate, from BayerCorp.) in a ball mill. The mixture was then passed through a metalfilter having an average opening size of about 3 μ m. The filteredmixture was then uniformly applied onto a flexible non-magnetic filmbase (for example, composed of polyethylene terephthalate) of 23 μ mthickness in such a coating amount that the coated film became about 35μm thick after drying (i.e., the dried applied coating had a thicknessof about 12 μm). The polyethylene terephthalate films coated with theaforesaid filtered compositions or mixtures were dried, calendered andheated at a temperature of 60° C. for 24 hours to cure the resins. Theso-produced films were then cut into 1/4 inch wide ribbons or tapes. Themagnetic recording tapes of inventive samples Nos. 1 to 6 in Table Ibelow were all prepared in the foregoing manner.

The "Q" sound, the dynamic friction coefficient, μd, the rubbed-offamount of magnetic powder and the splicing-ability of the foregoingexemplary tapes were measured by the following methods:

(1) "Q" sound (fluttering of magnetic recording tape) The abovedescribed 1/4 inch tapes were loaded into a professional tape recorder,model ES manufactured by Sony Corp., and were run therein underdifferent conditions. The obtained results were evaluated and recordedin terms of six grades on the basis of the following standards whereingrade (-5) represents the worst and grade (0) represents the best.

    ______________________________________                                                 Back-tension           Fluttering of                                          (Load condi-                                                                              Impedance  Tape                                          Grade    tion)(g.cm) Roller     ("Q" sound)                                   ______________________________________                                        -5       300         moved      generated                                     -4       840         moved      generated                                     -3       840         stopped    generated                                     -2       840         stopped    generated at times                            -1       840         stopped    scarcely                                                                      generated                                      0       840         stopped    not generated                                 ______________________________________                                    

(2) Dynamic friction coefficient, μd. As shown in FIG. 1, a tape 1 of1/4 inch width was contacted with the periphery of a brass cylinder 2over about a quadrant of the peripheral surface of such cylinder. Oneend of the tape 1 was horizontally fixed to a support means 3 and aweight W of 100 grams was fixed to the other free end of the tape 1 toprovide a given tension on the latent portion of the tape. The cylinder2 was then controllably rotated at such a rate that the peripheral speedthereof was 0.75 mm/sec relative to the tape. Tension gauges T₁ and T₂were arranged on the tape 1 between the weight W and the cylinder 2 andbetween the cylinder 2 and the support means 3 respectively as shown andtensions t₁ and t₂ were then noted. The dynamic friction coefficient,μd, was then calculated by the following equation:

    μd =2/π  log.sub.e t.sub. 2/ t.sub.1

(3) Rubbing-off amount of magnetic powder. A standard lapping or sandingtape No. 800, produced in accordance with JIS (Japanese IndustrialStandards) R 6001 and JIS R 6253 was wound on a roller. An exemplary 1/4inch width tape produced as above was hung over the roller under acompressive force of 100 grams and slid back and forth over such rollerfor a length of 10 cm. The decrease in weight of the abraded exemplarytape was measured in milligrams. The decrease in weight corresponded tothe rubbed-off amount of powder.

(4) Splicing-ability.

Exemplary tape produced as above was wound on a roller and maintained ata temperature of 45° C. under relative humidity of 80% for three days.Under such conditions, some of the lubricant within the magnetic layerof the tape was transferred to the back surface of the next layer oftape within the roll. Then, as shown in FIG. 2, an adhesive tape 5 (forexample, commercially available under the trade designation "41" fromthe Minnesota Mining and Manufacturing Corp.) of 1/4 inch width waspressed at a pressure of about 2 kilograms onto the back surface of thetape (which is opposite to the magnetic layer 4 of the tape 1). The endof the adhesive tape 5 was then lifted from tape 1 and was pulled in thedirection F shown by the arrow on FIG. 2. The pulling force F at whichthe adhesive tape 5 began peeling from the tape was measured in gramsand is tabulated below. The larger the pulling force F, the better thesplicing-ability of the particular tape.

The results obtained was inventive samples Nos. 1 to 6 produced as aboveare set forth in Table I below, along with comparative samples Nos. 1through 7, which were prepared essentially identically to the inventivesample except that different types of organopolysiloxanes, identified inthe Table, were utilized.

                                      TABLE I                                     __________________________________________________________________________            Organopolysiloxane                                                           Number                                Dynamic                                                                             Rubbed-off                 Inventive                                                                            of                                    Friction                                                                            Amount                                                                              Splicing-            Sample silicon                        "Q"    Coefficient                                                                         Power ability              No.    atoms           R   n   R'  m  Sound  μd (μg)                                                                             (g)                  __________________________________________________________________________    1      3     linear trisiloxane                                                                      CH.sub.3                                                                          1   C.sub.8 H.sub.17                                                                  2/3                                                                              0      0.278 59    89                   2      3     "         C.sub.2 H.sub.5                                                                   1   C.sub.15 H.sub.31                                                                 2/3                                                                              -1     0.272 61    80                   3      3     "         C.sub.3 H.sub.7                                                                   1   C.sub.17 H.sub.33                                                                 2/3                                                                              0      0.263 72    83                   4      5     cyclopentasiloxane                                                                      CH.sub.3                                                                          3/5 C.sub.8 H.sub.17                                                                  2/5                                                                              0      0.265 61    88                   5      5     "         C.sub.2 H.sub.5                                                                   3/5 C.sub.15 H.sub.31                                                                 2/5                                                                              0      0.263 64    84                   6      5     "         C.sub.3 H.sub.7                                                                   3/5 C.sub.17 H.sub.33                                                                 2/5                                                                              0      0.271 70    91                   Comparative                                                                   Sample No.                                                                    1      3     linear trisiloxane                                                                      CH.sub.3                                                                          1   C.sub.6 H.sub.13                                                                  2/3                                                                              -5     0.430 30    80                   2      3     "         C.sub.6 H.sub.23                                                                  1   C.sub.11 H.sub.23                                                                 2/3                                                                              -1     0.288 70    20                   3      3     "         C.sub.2 H.sub.5                                                                   1   C.sub.18 H.sub.37                                                                 2/3                                                                              unmeasur-                                                                            --    --    --                                                         able**                                  4      5     cyclopentasiloxane                                                                      CH.sub.3                                                                          3/5 C.sub.6 H.sub.13                                                                  2/5                                                                              -5     0.431 82    94                   5      5     "         C.sub.6 H.sub.13                                                                  3/5 C.sub.15 H.sub.31                                                                 2/5                                                                              -1     0.289 73    14                   6      5     "         CH.sub.3                                                                          3/5 C.sub.18 H.sub.37                                                                 2/5                                                                              unmeasur-                                                                            --    --    --                                                         able**                                  7      Methylphenylsilicone           -5     0.440 350   21                          fluid*                                                                 __________________________________________________________________________     *Commercially available under the Trade Name "KF 54" from Shin-Etsu           Chemical Co., Ltd., Japan                                                     **Blooming or bleeding occurred                                          

As is apparent from Table I, inventive sample Nos. 1 to 6 wherein therespective organopolysiloxane lubricant included an R having 1 to 3carbon atoms and an R' having 8 to 17 carbon atoms, respectively,exhibited superior valves in "Q" sound, dynamic friction coefficient,rubbing-off amount of magnetic powder and splicing-ability. On the otherhand, the results for the comparative samples Nos. 1 to 7 shown in TableI, clearly indicates that when either the R portion of anorganopolysiloxane has more than 5 carbon atoms or when the R' portionof an organopolysiloxane has less than 7 or more than 17 carbon atoms,substantially inferior valves for the measured properties were attained.

EXAMPLE II

In this exemplary embodiment of the invention, eight inventive samples,Nos. 7 to 14 were prepared substantially as described in Example I.However, in this embodiment, the organopolysiloxane in all samples was alinear trisiloxane which contained 3 silicon atoms, had an R which wasCH₃, had an R' which was C₁₅ H₃₁, had an m equal to 2/3 and had an nequal to 1. The amount of the foregoing organopolysiloxane was varied,as shown in Table II below, in each of the samples. The respectivesample tapes were prepared essentially in the manner described forExample I. These samples were then tested for their dynamic frictioncoefficient and rubbing-off amount of powder as set forth above and theresults were tabulated in Table II. A comparative sample, No. 8, whereinno organopolysiloxane was added into an otherwise identical compositionwas also tested and the results are shown in Table II.

                  TABLE II                                                        ______________________________________                                                   Added Lubri-                                                                             Dynamic    Rubbing-off                                  Inventive  cant Amount                                                                              Friction   Amount of                                    Sample     (parts     Coefficient                                                                              Powder                                       No.        by weight) (μd)    (μg)                                      ______________________________________                                         7         0.5        0.281      58                                            8         1.0        0.275      60                                            9         2.0        0.271      62                                           10         3.0        0.270      68                                           11         4.0        0.267      71                                           12         5.0        0.266      79                                           13         6.0        0.263      80                                           14         7.0        0.263      121                                          Comparative                                                                   Sample No.                                                                     8         0          0.448                                                   ______________________________________                                    

As is apparent from the foregoing Table II, the inventive sample Nos. 7to 13 which contained 0.5 to 6.0 parts by weight of the lineartrisiloxane showed acceptable and/or superior values both in dynamicfriction coefficient and in the rubbing-off amount of powder. Inventivesample No. 14 showed an acceptable dynamic friction coefficient but hadan impractical powder loss during the rubbing-off test while comparativesample No. 8 had an unacceptably high dynamic friction coefficient.

EXAMPLE III

In this exemplary embodiment of the invention, seven inventive samples,Nos. 15 through 21, were prepared in an essentially identical manner tothat described in Example I. In this embodiment, the organopolysiloxanesutilized were linear polysiloxanes which contained from 2 to 8 siliconatoms per molecule as indicated in Table III below, had an R which wasC₃ H₇, had an R' which was C₁₇ H₃₃ and had n and m values as stated inTable III. The respective sample tapes were prepared in the manner setforth above and were tested for the tabulated results as indicatedearlier. Comparative samples, Nos. 9 and 10 were similarly prepared andtested. Comparative sample No. 9 contained an organosilane, (CH₃)(C₃H₇)(C₁₇ H₃₃ C00)₂ Si and comparative sample No. 10 contained anorganopolysiloxane which had 9 silicon atoms per molecule thereof, hadan R which was C₃ H₇, had an R' which was C₁₇ H₃₃ and had an n and mvalues indicated in Table III.

                                      TABLE III                                   __________________________________________________________________________            Organopolysiloxane                                                                            Dynamic                                                                             Rubbing-off                                             Number of       Friction                                                                            Amount Of                                                                            Splicing-                                Inventive                                                                             Si Atoms in "Q" Coefficient                                                                         Powder Ability                                  Sample No.                                                                            Lubricant                                                                           n  m  Sound                                                                             (μd)                                                                             (μg)                                                                              (g)                                      __________________________________________________________________________    15      2     1  1  0   0.263 98     93                                       16      3     2/3                                                                              1  0   0.265 83     97                                       17      4     3/4                                                                              3/4                                                                              0   0.262 86     83                                       18      5     4/5                                                                              3/5                                                                              0   0.271 89     84                                       19      6     5/6                                                                              3/6                                                                              0   0.269 93     97                                       20      7     5/7                                                                              4/7                                                                              0   0.275 94     88                                       21      8     5/8                                                                              5/8                                                                              -1  0.281 90     85                                       Comparative                                                                   Sample No.                                                                     9      1     1  2  -2  0.321 201    92                                       10      9     8/9                                                                              6/9                                                                              -3  0.302 153    101                                      __________________________________________________________________________

As can be seen from the above results, the dynamic friction coefficient,μd, the "Q" sound and the rubbed-off amount of powder was sufficientlysmall for all of the inventive samples tested. On the other hand, thefriction coefficient, μd, was too large, some "Q" sound was generatedand the rubbed-off amount of powder was too large for comparative sampleNos. 9 and 10 so that such magnetic tapes are not preferred orpractical.

EXAMPLE IV

In this embodiment of the invention, eighteen inventive samples, Nos. 22through 39 were prepared in an essentially identical manner to thatdescribed in Example I. The various organopolysiloxanes utilized ininvention samples Nos. 22 through 39 are shown in Table IV below. Therespective sample tapes were prepared and tested as indicated earlierand the results are tabulated in Table IV.

                                      TABLE IV                                    __________________________________________________________________________    Organopolysiloxane              Dynamic                                                                             Rubbing-off                             Inventive                                                                          Number of                  Friction                                                                            Amount of                                                                           Splicing-                         Sample                                                                             Si Atoms in            "Q" Coefficient                                                                         Powder                                                                              ability                           No.  Lubricant                                                                           R      n  R'  m  Sound                                                                             (μd)                                                                             (μg)                                                                             (g)                               __________________________________________________________________________    22                1/3    4/3                                                                              0   0.265 83     83                               23                2/3    3/3                                                                              0   0.263 84     91                               24   3     CH.sub.3                                                                             3/3                                                                              C.sub.8 H.sub.17                                                                  2/3                                                                              0   0.271 86    102                               25                4/3    1/3                                                                              0   0.281 85    103                               26 27 28 29 30                                                                      4                                                                                   ##STR4##                                                                            1/4 2/4 3/4 4/4 5/4                                                              C.sub.11 H.sub.23                                                                 5/4 4/4 3/4 2/4 1/4                                                              0 0 0 0 0                                                                         0.273 0.263 0.265 0.269 0.273                                                       84 83 86 84 85                                                                      80 102 93 86 101                  31                1/5    6/5                                                                              0   0.265 93     42                               32                2/5    5/5                                                                              0   0.273 94     83                               33                3/5    4/5                                                                              0   0.263 80    103                               34   5     C.sub.4 H.sub.9                                                                      4/5                                                                              C.sub.17 H.sub.33                                                                 3/5                                                                              0   0.271 83     93                               35                5/5    2/5                                                                              0   0.273 84    101                               36                6/5    1/5                                                                              -2  0.213 94    111                               37   3     --     0  C.sub.8 H.sub.17                                                                  5/3                                                                              0   0.273 92     15                               38   4     --     0  C.sub.11 H.sub.23                                                                 6/4                                                                              0   0.265 93     23                               39   5     --     0  C.sub.17 H.sub.33                                                                 7/5                                                                              0   0.264 91     19                               __________________________________________________________________________

As is apparent from the tabulated results in Table IV above, the dynamicfriction coefficient, μd, was satisfactorily small, little, if any, "Q"sound was generated and the rubbed-off amount of powder was low for allof the invention samples in this embodiment.

Attention is directed to the fact that in the above invention samples,the magnetic tapes which contained an organopolysiloxane wherein theratio n/m (i.e., ratio of alkoxy groups to acyloxy groups) ranged from0.2 to 5 exhibited a superior splicing-ability.

EXAMPLE V

In this embodiment of the invention, no organopolysiloxane was addedinto the magnetic composition earlier described. Instead, the earlierdescribed magnetic composition was coated onto polyethyleneterephthalate films and processed as earlier described and after themagnetic layer was formed, a top coat of the various organopolysiloxanesidentified in Table V below was applied onto such magnetic layers.

The base films were coated with the magnetic composition, dried andcalendered as described in Example I. Then, liquid top-coat solutionscomprised of 3 grams (per liter of solvent) of an organopolysiloxaneshown in Table V, and methylethylketone were prepared and were eachapplied onto the free surface of a magnetic layer of each inventivesample, Nos. 40 through 42 and to comparative sample Nos. 11 and 12. Theamount of applied top coating in each case was about 30 milligrams/m²,calculated relative to the organopolysiloxane. The coated magneticlayers were then dried to prepare sample tapes which were then tested asset forth earlier. The results are tabulated in Table V below:

                                      TABLE V                                     __________________________________________________________________________                                     Dynamic                                                                             Rubbing-off                                   Number of                 Friction                                                                            Amount of                                                                           Splicing-                        Inventive                                                                            Si Atoms in           "Q" Coefficient                                                                         Powder                                                                              ability                          Sample No.                                                                           Lubricant                                                                           R   n   R'  m   Sound                                                                             (μd)                                                                             (μg)                                                                             (g)                              __________________________________________________________________________    40     5     CH.sub.3                                                                          2/5 C.sub.16 H.sub.33                                                                 1   0   0.268  56   89                               41     3     C.sub.2 H.sub.5                                                                   1   C.sub.16 H.sub.33                                                                 2/3 0   0.265  60   92                               42     2     CH.sub.3                                                                          1   C.sub.14 H.sub.29                                                                 1   0   0.280 100   85                               Comparative                                                                   Sample No.                                                                    11     Methylphenylsilicone fluid*                                                                         -3  0.365 360   30                               12     Dimethylsilicone fluid**                                                                            -2  0.421 380   20                               __________________________________________________________________________      *Available under the trade name "KF 54" from Shin-Etsu Chemical Co.,         Ltd., Japan                                                                    **Available under the trade name "KF 96" from Shin-Etsu Chemical Co.,        Ltd., Japan                                                              

As is apparent from the tabulated results in Table V, the sample tapeswhich were coated (top-coated) with the specified organopolysiloxanesexhibited values in the "Q" sound generated, the dynamic frictioncoefficient, the rubbed-off amount of powder and splicing-ability whichwere at least as good as the values for such properties exhibited byearlier sample tapes in which the organopolysiloxanes were added intothe magnetic layer.

The solvent in preparing a liquid lubricant top coating composition maybe other than methylethylketone mentioned above, for example,dichlorodifluoromethane, CCl₂ F₂, commercially available under the tradename "Freon-12" from E. I. du Pont de Nemours & Co. or isopropylalcoholor some other compatible solvent may be used in formulating thelubricant top coat composition.

Although several specific embodiments of the invention have beendescribed in the preceding Examples, it should be understood thatmodifications may be made to such embodiments without departing from theinvention. For example, two or more organopolysiloxanes as definedhereinabove may be used in combination on or in a magnetic layer of amagnetic recording medium.

As is apparent from the foregoing specification, the present inventionis susceptible of being embodied with various alterations andmodifications which may differ particularly from those that have beendescribed in the preceding specification and description. For thisreason, it is to be fully understood that all of the foregoing isintended to be merely illustrative and is not to be construed orinterpreted as being restrictive or otherwise limiting of the presentinvention, excepting as it is set forth and defined in thehereto-appended claims.

We claim as our invention:
 1. In a magnetic recording medium having anon-magnetic base and a magnetic layer formed thereon containingmagnetizable particles dispersed in a resinous binder, the improvementcomprising wherein:said magnetic layer has an organopolysiloxane addedinto such layer, said organopolysiloxane having an average unit formula:

    (CH.sub.3)(RO).sub.n (R'COO).sub.m SiO.sub.(3-n-m)/2

wherein R is a monovalent hydrocarbon group having from 1 to 5 carbonatoms, R' is an aliphatic monovalent hydrocarbon group having from 7 to17 carbon atoms; n is zero or a positive number; m is a positive numberwith the proviso that n + m is less than 3 and the number of Si atoms ina molecule of such organopolysiloxane ranges from 2 to 8; saidorganopolysiloxane being present within said magnetic layer in an amountranging from about 0.5 to 6 parts by weight per 100 parts by weight ofmagnetizable particles in said magnetic layer.
 2. In a magneticrecording medium as defined in claim wherein said magnetic layerincludes at least two of said organopolysiloxanes in combination, eachas defined by the average unit formula set forth in claim 1
 3. In amagnetic recording medium as defined in claim 1 wherein saidorganopolysiloxane contains an R' which is C₁₇ H₃₃.
 4. In a magneticrecording medium as defined in claim 1 wherein a molecule of saidorganopolysiloxane contains 3 silicon atoms.
 5. In a magnetic recordingmedium as defined in claim 1 wherein said organopolysiloxane has a ratioof n/m ranging from 0.2 to
 5. 6. In a magnetic recording medium asdefined in claim 1 wherein said magnetic layer includes a materialselected from the group consisting of antistatic agents, dispersingagents and mixtures thereof.
 7. In a magnetic recording medium having anon-magnetic base and a magnetic layer formed thereon containingmagnetizable particles dispersed in a resinous binder, the improvementcomprising wherein:said magnetic layer has an organopolysiloxane coatedonto the free surface of such layer, said organopolysiloxane having anaverage unit formula:

    (CH.sub.3)(RO).sub.n (R' COO).sub.m SiO.sub.(3-n-m)/2

wherein R is a monovalent hydrocarbon group having from 1 to 5 carbonatoms, R' is an aliphatic monovalent hydrocarbon group having from 7 to17 carbon atoms; n is zero or a positive number, m is a positive numberwith the proviso that n + m is less than 3 and the number of Si atoms ina molecule of such organopolysiloxane ranges from 2 to 8; saidorganopolysiloxane being coated onto said magnetic layer in an amountranging from about 30 to 1500 milligrams per square meter of saidmagnetic layer.
 8. In a magnetic recording medium as defined in claim 7wherein said magnetic layer includes at least two of saidorganopolysiloxanes in combination, each as defined by the average unitformula set forth in claim
 1. 9. In a magnetic recording medium asdefined in claim 7 wherein said organopolysiloxane contains an R' whichis C₁₇ H₃₃.
 10. In a magnetic recording medium as defined in claim 7wherein a molecule of said organopolysiloxane contains 3 silicon atoms.11. In a magnetic recording medium as defined in claim 7 wherein saidorganopolysiloxane has a ratio of n/m ranging from 0.2 to
 5. 12. In amagnetic recording medium as defined in claim 7 wherein said magneticlayer includes a material selected from the group consisting ofantistatic agents, dispersing agents and mixtures thereof.